In recent years, multispectral imaging has been demonstrated to be a useful method of evaluating features of plants, identifying defects in produce, or providing feature identification of other materials such as plastics and wood. Prism based multispectral cameras use a color separating prism to split an image into multiple images, each in a specific spectral band. The spectral bands can be in the ultraviolet, visible, and near infrared spectral regions. For plant imaging, color-infrared imaging is commonly utilized with imaging bands in the green, red, and near infrared regions. Satellite based systems are often used for this type of imaging in remote sensing applications. The French-built SPOT satellite acquires images in the green, red, and near infrared bands. The U.S.-built LANDSAT satellite acquires images in four spectral bands (blue, green, red, and near infrared). In many cases, terrestrial multispectral imaging has been accomplished using clusters of cameras, each filtered for a specific spectral region. These cluster cameras have proven difficult to align and maintain. Alternatively, multispectral imaging can be accomplished by using a common objective lens and a color separating prism to separate spectral bands. The primary applications for multispectral imaging require a rugged camera not affected by temperature and vibration. Applications include aerial imaging, produce sorting, and advanced surveillance.
Color separating prisms and lenses specifically designed to work with these color separating prisms are used extensively in electronic news gathering (ENG) cameras. Many color separating prisms have been developed and patented. A common color separating prism geometry is described in U.S. Pat. No. 4,084,180, issued Apr. 11, 1978. The dichroic image separating coatings can be selected to separate spectral image channels throughout the ultraviolet, visible, and near infrared spectra.
The color separating prism introduces spherical aberrations and chromatic aberrations in the resulting images. Lenses designed specifically to be used with color separating prisms are described, for example in U.S. Pat. No. 5,760,969, issued Jun. 2, 1998. The color separating prism over-corrects spherical aberration. To compensate for this effect, lenses for use with these prisms are designed to under-correct spherical aberration thereby causing the two effects to cancel each other. Similarly these lenses are designed to compensate for longitudinal chromatic aberrations introduced by the prism glass material.
Commercial ENG lenses are corrected for chromatic aberrations in the visible spectral region. Outside of the visible spectral region commercial ENG lenses commonly exhibit longitudinal chromatic aberrations that result in a shift of the focal plane location and differences in the size of the resulting images as a function of wavelength. In addition many ENG lenses are highly absorptive in the near infrared rendering them ineffective for multispectral imaging, so either a custom lens or a commercial lens with a long back focal length adapted for use with a multispectral color separating prism is required.
Lenses for use with Single Lens Reflex (SLR) cameras are designed with a long flange focal length to accommodate the pentaprism view finder. These SLR lenses are not designed to correct for prism induced aberrations so the use of SLR lenses with color separating prisms results in undesirable aberrations. However SLR lenses have a flange focal length adequate to physically accommodate a color separating prism between the lens and image plane. Similarly mid and large format still camera lenses have long flange focal lengths but are not designed to compensate for the presence of a color separating prism.
Multispectral imaging is best accomplished when the images acquired by each channel's image sensing device are identical in size. It is desirable that the image sensing pixels in each image channel see exactly the same geometric region in the field-of-view so that the images are exactly registered. Achromat ENG lenses and color separating prisms provide good image registration in the visible portion of the spectrum but exact image registration is not feasible over a broad spectral range that spans beyond the visible region.
In existing 3-CCD cameras, the imaging array is bonded directly to the prism. U.S. Pat. No. 4,916,529, issued Apr. 10, 1990, describes a 3-CCD color separating prism. In this type of configuration, the thickness of the trim filters must be very exactly controlled and, once bonded, cannot be interchanged. For specific applications, i.e., primary color (red, green blue) imaging this is acceptable. Multispectral imaging often requires trim filters specific for the application. Commercial bandpass filters used for trim filters in multispectral cameras often do not have accurately controlled thickness.
In existing 3-CCD cameras, the thermal waste heat from the imaging sensors and electronics is conducted into the color separating prism. Temperature gradients in the prism can cause image distortion and stresses in the bond joints. The glass prism is a poor thermal conductor so the imaging array temperature is often elevated significantly above ambient. Imaging arrays generally have a doubling in noise for every 10.degree. C. rise in temperature. Linear arrays operate at high pixel clock rates, have large photosite areas, and large pixel counts that result in particularly high heat dissipation.
Existing cameras dissipate thermal waste heat via free convection using a perforated camera case or by forced convection using a fan. Both of these approaches provide a direct path for dirt to enter the camera electronics and optics, exposing the camera components to contaminates. U.S. Pat. No. 5,221,964, issued Jun. 22, 1993, shows printed circuit boards mounted on standoffs, with no good thermal conduction path to the outside environment.
The following patents are also of some degree of relevance: U.S. Pat. No. 4,444,472, issued April, 1984, U.S. Pat. No. 4,789,891, issued December, 1988, U.S. Pat. No. 5,134,468, issued July, 1992, U.S. Pat. No. 5,870,228, issued February, 1999, and U.S. Pat. No. 5,889,555, issued March, 1999.
The patents do not teach or suggest the invention disclosed and claimed herein.